When measuring enzymatic activity of trypsin, generally an aqueous solution of trypsin (an enzyme solution), a buffer solution, and a substrate solution are prepared separately, and these three solutions are admixed so as to generate enzyme reaction. When trypsin is dissolved and stored in a purified water, the amount of trypsin often decreases due to autolysis. In order to prevent this phenomenon, the pH of an aqueous trypsin solution is usually adjusted to a range in which the trypsin is inactive but not deactivated. For example, trypsin may be dissolved in a hydrochloric acid aqueous solution (1 mmol/l). Also, in order to further increase the stability of trypsin, calcium ions may be added to the hydrochloric acid aqueous solution.
However, in the above-mentioned conventional method of mixing three solutions, additional labor is required for preparing the solutions and for measuring the enzyme reaction. Furthermore, when measuring the enzymatic activity of trypsin with an automatic analyzer in a clinical test or the like, which requires processing a large amount of sample, it is desired to integrate such a three-solution system into a two-solution system. Therefore, dissolving a substrate in a buffer solution may be considered, but there is a problem that .alpha.-benzoyl-arginine-p-nitroanilide (BAPNA) or the like generally used as a synthetic substrate for trypsin is unstable in a buffer solution.
Furthermore, in the above-mentioned automatic analyzer or the like, it is desired to prepare a dry reagent by drying a reagent used for measuring the enzymatic activity of trypsin, and adhering it to a test piece or the like. However, when trypsin is stabilized using hydrochloric acid as mentioned above, the hydrochloric acid volatilizes during the drying step, so that the stability of trypsin is reduced. Although other non-volatile acids may be used in place of hydrochloric acid, there is a possibility that acid concentration is increased in the drying step, resulting in deactivation of trypsin, if such acids are used.